Gaseous discharge device



Oct. 8, 1940. c. G. SMITH 7 2,217,185

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?aiented Get. 8, 1949 UNITED STATES PATENT OFFICE GASEOUS DISCHARGE DEVICE Charles G. Smith, Mcdford, Mass., assignor, by

mesne assignments, to Raytheon Manufacturing Company, Newton, Mass., a corporation of This invention relates to gaseous conduction devices generally, such as rectifiers, amplifiers, oscillators, converters between D. C. and A. C., etc., and in certain aspects is particularly useful in conjunction with the inventions described in copending applications which have now matured into Patents 1,617,171, 1,617,179, 1,816,619, 1,878,- 338, 1,929,122, 2,137,198, 2,201,817, 2,201,819, and also copendin'g application Serial No. 504,611, filed as a continuation of application Serial No. 76,793.

In one aspect the invention involves the discovery that a hot cathode operated in the presence of certain easily ionizable gases having a strong afiinity for the cathode material, such as the vapor of an alkaline earth metal (barium, strontium and calcium), is a powerful source of electrons. For example, in an atmosphere of calcium vapor at a pressure of the order of 10 mm., tungsten emits a large thermionic current at temperatures below 1800 C. and nickel at about 1000 C.

In another aspect the invention comprises means for maintaining a supply of the vapor at the active surface of the cathode. This may be accomplished, for instance, by incorporating an alkaline earth metal within the device and heating the metal to vaporizing temperature, thereby generating the vapor within the tube or container. The electron emitting surface is preferably confined by an enclosure having only a restricted opening for the cathode-anode discharge, the enclosure comprising either thewalls of a hollow cathode or an additional part surrounding the cathode and the alkaline earth metal vapor is preferably confined substantially to the interior of this enclosure, as for example by generating the vapor within the enclosure by heat derived from the cathode-anode discharge or auxiliary heating means or both.

The foregoing and other objects of the invention will be best understood from the following description of exemplification thereof shown diagrammatically in Figs. 1 to 5 of the accompanying drawing.

In the space discharge device illustrated in Fig. 1, T represents the tube or vessel which may be of any suitable construction and A1 and A2 plate anodes. Cooperating with the anodes is a partially open hollow electrode structure comprising an enclosure K having an extended interior thermionically-emitting cathode surface provided by C, a tubular electron emitter, constituting a hollow housing member extending into the interior of enclosure K and segregating the space in the interior of C from the space of said enclosure. The housing member C has a thermionically-emitting cathode surface on its exterior within the enclosure K. A heater H is disposed in the hollow housing member C for 5 heating the cathode surface to a temperature of thermionic electron emission during operation. The enclosure K also has a restricted opening S for the electrical discharge passing between the interior of K and the other electrodes or anodes A1 and A2, M the vapor source, such as calcium or an alloy of calcium disposed in the bottom of an elongate tubular appendix to K, and R a bond for conducting heat from the tubular electron emitter C to said appendix. Hg is a drop of mercury for maintaining mercury vapor throughout the tube. As illustrated the device is connected to a rectifying circuit including a load L, the filament H being heated by an auxiliary coil connected to the heater H and enclosure K in series.

In operation the filament H is heated sufficiently to vaporize the metal M to the desired pressure. When employing calcium for example the temperature of the metallic calcium may be approximately 500" C. The interior of enclosure K is preferably heated to a higher temperature (e. g. 1000 C. when using calcium), this difference in temperature within K being due in part to the long narrow appendix, in part to the location of the heater, and in part to the electrical discharge through the upper part of the enclosure. Calcium vapor has such strong afiinity for cathode metals (e. g. tungsten) that it is not rapidly driven off from C at temperatures affording large thermionic emission. For example, the vapor pressure of the calcium on C may not be greater than 10 mm. even when C is hot enough for large thermionic currents, whereas the vapor pressure of pure calcium at such temperatures would be thousands of times as much. Thus the electron emission from C is greatly enhanced by the calcium. The source of activating material M, which is contained in its appendix or compartment communicating with the cathode surface, is a metal supply of the alkaline earth metal, spaced from the cathode surface, for supplying the metal to the cathode surface to enhance and maintain copius electron emission from said surface as described above.

The alkaline earth metalsdescribed above are substantially non-vaporizable at the normal operating temperature of the vessel T. However, when employing a low vapor pressure within K (e. g. calcium vapor at 10* mm.) the escape of the vapor through opening S (which may be 1':

inch in diameter) is slow even when unimpeded. By filling the space outside the enclosure K with an inert gas as disclosed in certain of the aforesaid prior applications, the vapor can escape only by diifusion which is very slow. Moreover, by employing a considerable cathode-anode discharge (e. g. several amperes per square cm.) I believe that the vapor may be retained within the enclosure by an electric pumping action. In accordance with my present understanding of the theory of the operation of my device, this action is as follows:

The mercury vapor (at a pressure of the order of magnitude of 10 microns of mercury, e. g.) serves as an ionizable gaseous medium to conduct current through the opening S, the positive ions formed in the mercury vapor permitting large electronic discharge through S with the application of only a few volts (e. g. ten volts). Thus the pressure of the gas in the enclosure K, which gas in this instance comprises mercury vapor and alkaline earth metal vapor, is at a pressure sufliciently high to secure a discharge between the cathode surface and the anodes A1 and A2 at a voltage of the order of the ionization voltage of the gas. Under these conditions the cathode voltage drop is low and likewise of the order of the ionization voltage of the gas. Herein a discharge with a low cathode voltage drop will be termed a low-voltage discharge. Since the ionization voltage of calcium vapor is 6 volts while that of mercury vapor is 10.4 volts the calcium vapor is intensely ionized by the heavy discharge through the restricted opening S and the calcium ions are drawn back into the enclosure. When the discharge is passing under such conditions the spectrum of calcium is strong inside K and absent outside.

From the foregoing it will be evident that the material M is heated independently of the cathode-anode discharge (although it may also derive some heat from the discharge) the appendix being too long and narrow to permit the discharge to pass directly to the material. Consequently the vapor pressure of the calcium. may be maintained sufliciently low to prevent substantial loss through opening S throughout a wide range of operating currents. I believe that by confining the calcium vapor to the enclosure K'a pressure differential is maintained between the cathode and anode, the active vapor having a higher pressure near the electron emitting surface, that is, within the enclosure. During operation the alkaline earth metal vapor may be sufficient in amount to constitute a gaseous filling within the enclosure K at a pressure high enough to secure a discharge between the anodes A1 and A2 and the interior of K.

The form of the invention illustrated in Fig. 2 differs from that of Fig. 1 in that the cathode C is in the form of a rod of nickel, tungsten or other suitable material and is spaced from the enclosure K so that the main discharge through opening S between cathode C and anode A may be started by first producing an auxiliary discharge between C and K at a voltage of the order of volts (A. C.). This auxiliary discharge heats K and C until the calcium M begins to vaporize, whereupon the action is similar to that described in relation to Fig. 1. The regulation of the circuit supplying the discharge between K and C is such that its voltage drops rapidly as the current between K and C increases. Thus K heats until it becomes an appreciable emitter of electrons whereupon no further rise in temperature takes place since the voltage between C and K has dropped to a low value and the regulation of the circuit prevents further increase in temperature, whereby the temperature of K is sensibly constant and selfregulating. Consequently, the appendix containing the material M is designed to be heated to a predetermined temperature (about 500 C. when M is calcium) by a container K of constant temperature.

The use of an inert gas in the tube restrains the escape of the calcium vapor through opening S by virtue of the diffusion phenomenon and in a device such as illustrated in Fig. 2 the gas may for example be argon or neon at approximately two centimeters pressure.

In Fig. 3 which illustrates an application of the invention to a three-electrode tube of the audion type, B is a cathode filament, I a cylindrical grid, and J a cup-shaped anode serving as a vapor enclosure, each supported upon its respective lead or leads, the filament having two leads as usual. The lower end of the cup-shaped anode has a closure provided with three tubular openings for the leads to the filament and grid. The vapor yielding material (e. g. calcium) may be located at N where it is vaporized chiefly by radiation from the filament B. By making said tubular openings narrow the escape of calcium vapor is minimized and by making them extend a considerable distance into the enclosure J they are heated sufficiently to prevent the calcium from condensing therein. Disk bafiles may be mounted on the leads just outside the said openings to prevent condensation on the stem, the small amount of calcium which escapes through said openings condensing upon the cool baffles. If desired an inert gas may be incorporated in the tube as above described. This device may be used in any one of the many ways for which tubes of the audion or grid type are adapted.

In Figs. 4 and 5 the enclosure 0 is tubular and T-shaped. The lower end of the enclosure is closed and contains the calcium or other vapor material P. The horizontal ends of the enclosure are open and extending therethrough is a filament Q which heats the enclosure and vaporizes the calcium. The filament may be formed of tungsten and, when using calcium vapor at a pressure of the order of 10- mm. e. g., may be heated to approximately 1300 C. The plate anodes U and V are located on opposite sides of the enclosure. A drop of mercury W may be incorporated in the tube as above described.

In operating a device of the type illustrated in Figs. 4 and 5 either the filament Q or the enclosure 0 may be used as the cathode. When filament Q is connected as the cathode it emits a copious supply of electrons under the stated conditions and the discharge passes to the anodes through the open ends of the enclosure 0. When the enclosure is connected as a cathode it acts as a hollow cathode structure and emits chiefly from its interior electron-emitting surface by virtue of the heat derived from the filament, the calcium coated on its inner surface, and the ionized calcium vapor inside the enclosure. Calcium as well as the other alkaline earth metals and equivalent activating materials when used as a coating in this way emit substantial quantities of electrons at a relatively low temperature. The filament Q inside the hollow cathode structure 0 is directly exposed to the electron-emitting surface and can be independently heated to maintain the coating on said surface at temperature of electron emission to sustain a discharge between said surface and the other electrodes or anodes U and V.

The invention is not limited to the particular detail and arrangements described above, but its principles are applicable to many other constructions that will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad construction commensurate with the scope of the invention within the art.

I claim:

1. A space discharge device comprising a vessel containing a hollow electrode structure having an interior thermionically-emitting cathode surface and constituting an enclosure, another electrode in said vessel outside said enclosure, said enclosure having a hollow housing member ex- 1 tending into the interior of said enclosure to segregate the space in the interior of said member from the space of said enclosure, an alkaline earth metal vapor within said enclosure during operation, and a heater disposed in said hollow housing member for heating said surface to a temperature of electron emission.

2. A space discharge device comprising a vessel containing electrodes, including a partially open nollow electrode structure having an extended interior cathode surface and another electrode, means for heating said interior cathode surface to thermionic emission during operation, and an alkaline earth metal vapor sufficient in amount to constitute during operation a gaseous filling within the hollow electrode structure at a pressure high enough to secure a discharge between said other electrode and the interior of said electrode structure.

3. A space discharge device comprising a vessel containing electrodes, including a partially open hollow electrode structure having an extended interior cathode surface and another electrode, means for heating said interior cathode surface to thermionic emission during operation, and an alkaline earth metal vapor sufficient in amount to constitute during operation a gaseous filling within the hollow electrode structure at a pressure high enough to secure a discharge between said other electrode and the interior of said electrode structure, said pressure being greater than the pressure of said vapor outside of said hollow electrode structure during operation.

4. A space discharge device comprising a vessel containing electrodes, including a partially-open hollow electrode structure having an extended interior cathode surface to be heated to thermionic emission during operation, and another electrode and an atmosphere comprising a gas and analkaline earth metal vapor sufficient in amount to constitute during operation a gaseous filling within the hollow electrode structure at a pressure high enough to secure a low voltage discharge between said other electrode and the interior of said electrode structure.

5. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to be heated to thermionic emission during operation, and another electrode, and a gas adjacent said cathode surface at a pressure suffi- 'ciently high during operation to secure a lowvoltage discharge between said other electrode and said cathode surface, said gas comprising an alkaline earthmetal vapor.

6. A space discharge device comprising a vessel containing a hollow electrode structure constituting an enclosure, another electrode in said vessel outside said enclosure, said enclosure having a hollow housing member extending into the interior of said enclosure to segregate the space in the interior of said member from the space of said enclosure, said hollow housing member having a thermionically-emitting cathode surface on the exterior thereof within said enclosure, an alkaline earth'metal vapor within said enclosure during operation, and a heater disposed in said hollow housing member for heating said cathode surface to a temperature of electron emission.

7. A space discharge device comprising an envelope containing a hollow cathode structure containing an interior electron-emitting surface, another electrode, and a gas, said electron-emitting surface having a coating adapted to emit electrons at a relatively low temperature, and an element inside said hollow cathode structure directly exposed to said electron-emitting surface, to be independently heated to maintain said coating on said surface at a temperature of electron emission to sustain a discharge between said electron-emitting surface and said other electrode, the pressure of said gas being of the order of magnitude of ten microns of mercury during operation.

8. A space discharge device comprising a vessel containing electrodes, including a partially open hollow electrode structure having an extended interior cathode surface, means for heating said cathode surface to thermionic emission during operation, and an alkaline earth metal vapor at a substantial pressure within the hollow electrode structure, said pressure being greater than the pressure of said vapor outside of said hollow electrode structure during operation,

9. A space discharge device comprising a vessel containing electrodes, including a partially open hollow electrode structure having an extended interior cathode surface and another electrode, means for heating said surface to thermionic emission during operation, alkaline earth metal vapor and a gas suflicient in amount to constitute during operation a gaseous filling within the hollow electrode structure at a pressure high enough to secure a discharge between said other electrode and the interior of said electrode structure.

10. A space discharge device comprising a vessel containing electrodes, including a partiallyopen hollow electrode structure having an extended interior cathode surface to be heated to thermionic emission during operation, a coating on said surface, said coating comprising alkaline earth metal, and a supply of alkaline earth metal for supplying metal to said surface to enhance and maintain copious electron emission from said surface.

11. A space discharge device comprising a vessel containing electrodes, including a partially-open hollow electrode structure having an extended interior cathode surface to be heated to thermionic emission during operation, a coating on said surface, said coating comprising alkaline earth metal, a. supply of alkaline earth metal for supplying metal to said surface to enhance and maintain copious electron emission from said surface, and an ionizable gas filling.

12. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to support a discharge therefrom, and another electrode, a heater segregated from the discharge space, and a gas adjacent said cathode surface at a pressure sufliciently high during operation to secure a low-voltage discharge between said other electrode and said cathode surface, said gas comprising an alkaline earth metal vapor.

13. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to be heated to thermionic emission during operation, and another electrode, and a gas adjace'nt said cathode surface at a pressure sufficiently high during operation to secure a lowvoltage discharge between said other electrode and said cathode surface, said gas comprising an alkaline earth metal vapor, said cathode surface comprising a material having an affinity for said alkaline earth metal whereby said alkaline earth metal enhances and maintains the electron emissivity of said cathode surface.

14. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to be heated to thermionic emission during operation, a compartment communicating with said cathode surface, a metal supply in said compartment, said supply comprising alkaline earth metal for supplying said alkaline earth metal to said surface during operation to enhance and maintain copiouselectron emission from said surface.

15. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to be heated to thermionic emission during operation, a supply of activating material in said vessel, said material being substantially non-vaporizable at the normal operating temperature of said vessel, and means for supplying said material to said surface during operation to enhance and maintain copious electron emission from said surface.

16. A space discharge device comprising a vessel containing electrodes, including an electrode structure having an extended cathode surface to be heated to thermionic emission during operation, said surface having an activating material thereon, which is substantially non-vaporizable at the normal operating temperature of said vessel, and an additional supply of said material located in said structure and spaced from said surface for supplying said material to said surface during operation to enhance and maintain copious electron emission from said surface.

CHARLES G. SMITH. 

